RE: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint

specify cardinality(tableA xxx) dynamic_sampling(tableB,9)

With just dynamic sampling I get a good plan but I noticed the cardinality estimates for tableA were off, so I added the cardinality hint for tableA and the plan went back to the previous plan when not using dynamic_sampling.

(again version 10.2.0.4)

I'd provide some formatted details, but it doesn't seem I can send RTF to the list from our exchange server. I'll try to come up with a test case and see if I can mail it later using my gmail account.

Chris

-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Jonathan Lewis Sent: Friday, September 14, 2012 8:21 AM To: oracle-l_at_freelists.org
Subject: Re: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint

Which version are you on.
I've just done a couple of simple checks on 11.2.0.3 and there doesn't seem to be any unreasonable conflict.

Of course, if you specifiy table-level dynamic sampling and specify the single table cardinality for the same table then you have to expect one of the hints to "lose" - and in this case the cardinality hint overrides anything the dynamic sampling may have done.

Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com/all_postings

Author: Oracle Core (Apress 2011)
http://www.apress.com/9781430239543

• Original Message ----- From: <Christopher.Taylor2_at_parallon.net> To: <oracle-l_at_freelists.org> Sent: Thursday, September 13, 2012 9:33 PM Subject: Fun with the CARDINALITY and DYNAMIC_SAMPLING hint

| (Thanks to Jonathan Lewis, Greg Rahn (and others) I've been learning a
lot about tweaking the execution paths for a particular problematic SQL.
| A special thank you to you guys for all the blog posts (a tremendous
source of knowledge) that you write at Oracle Scratchpad and StructuredData!)
| I was playing around with the hints mentioned in the subject for a
particular problematic query we're working on and using dynamic_sampling I could reduce the performance from 25M lios to 5M lios (which I think is still probably too high for this complex DSS query).
|
| However, when I added the CARDINALITY hint, it seems to immediately
disallow the dynamic sampling hint (or something)?
|
| Anyone know if these 2 are compatible? It doesn't seem so.
|
| Also, as a "fun" learning experiment, I played around with cardinality
hints that were completely unrealistic such as 9999999999 (9 billion) - interesting explain plans doing things like that :) Performance not so great, but was still interesting to see how you can move tables around in the xplan based on what you know of them.
|
| Chris
|
|

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